High-temperature fuel cells are developed for operating temperatures between 650° C. and 1000° C. Different materials are used in dependence on the development goal. Cathodes of an SOFC fuel cell are known which are made of ceramic material. Interconnectors are also known which are made of metal.
A functional layer is to be understood as a layer which has to satisfy a defined function. A contact layer which takes over the electrically conductive function between the cathode and the interconnector within the fuel cell represents an example for such a functional layer. This functional layer on the one hand ensures an electrical contact between the two components with which this layer forms a connection at both sides with material continuity to both components as a consequence of a joining process.
Previously used materials for a functional layer between the cathode and the interconnector made from lanthanum manganite or lanthanum cobaltite are less sinter-active at temperatures of 950° C. or below, i.e. the required diffusion processes are too low to realize a good electrical contact and sufficient adhesion to boundary layers. It is also disadvantageous that the layers which form from these materials are very porous and can thereby not prevent a corrosion of the interconnector by air in the cathode space.
In accordance with DE 19710345 C1, a paste, in which a first phase is made of glass or of a glass ceramic material and a second phase is electrically conductive, is provided for the realization of the connection between the cathode and the interconnector. The paste as a rule comprises a similar material or the same material as the cathode. This paste is first applied to the interconnector. The interconnector and the adjacent cathode are subsequently heated with the paste to a temperature of up to 900° C. Temperatures above 900° C. are, however, damaging in the long term for the material of the interconnector since it is usually formed from a steel.
It is also customary to use a glass solder for the sealing of fuel cells. A glass solder used for this purpose likewise cannot withstand temperatures of 900° C., however.
It is also disadvantageous with this known paste that it is only electrically conductive subsequent to a sintering. As a rule it does not sinter or sinters too little at 900° C. The paste therefore has low electrical conductivity in the operation of a high-temperature fuel cell.
As is described in DE 19749004 C1, an electrically conductive connection was realized between a ceramic component and a metallic component with a sintered, electrically conductive paste between both components. This connection is established in that the paste is applied to a ceramic component (e.g. a ceramic cathode) at least at the points at which the second component (e.g. webs of an interconnector) should contact the ceramic surface. With the electrical conductivity of the paste being able to be improved by sintering. The surface with the sintered paste is ground until a planar surface is produced for the suitable contacting. If the component is an electrode-electrolyte unit with a thin cathode, the cathode is not abraded by the grinding due to the selected thickness of the paste.
The powder can be formed from a glass-metal combination or from a ceramic powder depending on the sintering temperature. For lower sintering temperatures (<800° C.), the paste, on the one hand, contains a powder made from the starting materials for glass and, on the other hand, pulverized silver in a volume ratio of 1:1. Instead of the silver, a silver oxide powder and/or a silver alloy in powder form can be used, in each case also together with silver powder. Since silver is very noble, it can even be used for contact layers in cathode spaces (spaces in which the cathodes are located) of a fuel cell between the interconnector and the cathode although an oxidizing atmosphere is present there due to the supplied air.
The contacts known from DE 19749004 generally have the following disadvantages:                Due to the high vapor pressure of silver at T>800° C., the contact layers on the basis of a silver-glass ceramic material do not have long-term stability.        It is also disadvantageous that the layers which are formed from such pastes cannot prevent poisoning the cathode by Cr evaporation from the interconnector due to the air flowing through the cathode space.        
In accordance with DE 19941282 C1, a mixture is used for the realization of an electrically conductive contact between an interconnector and a cathode of a fuel cell which comprises cuprates, CuO/cuprate compounds or silver/cuprate compounds and forms melts or part melts in the temperature range from 800-1400° C. Compounds having the following composition are preferably used as cuprates: (La,Sr)2CuO4−x, (La,Sr)CuO2.4+x, YBa2Cu3O7−x, Bi2Sr2Ca2Cu2O8+x or also Bi2(Sr,Ca)2CuO6+x or (Bi,Pb)2Sr2Ca2Cu3O10+x. In addition, mixtures of CuO and cuprates or of Ag and cuprates are also suitable as materials for the contact. In this respect, the proportions of CuO in the mixture can amount to up to 30% by weight or of Ag up to 10% by weight. The contacts known from DE 19941282 have the following disadvantages, however:                The contact layers on the basis of cuprate compounds have no long-term stability due to the high vapor pressure of the copper at T>850° C. (La,Sr)CuO2.4+x, for example, continuously shows phase conversions from the perovskite structure into the K2NiF4 structure after the sintering at temperatures >900° C. This phase conversion can have a very negative influence on the layer properties, which has a particular effect on a thermal cyclization. The K2NiF4 structure has a lower thermal coefficient of expansion, which results in incompatibilities with the other cell layers.        In addition, mechanical influences such as vibrations, pressure changes and tensions can also not always be compensated with the layers or a sufficiently large resistance against such influences cannot be achieved and the electrically conductive connection is also again impaired in an unwanted manner.        It is also disadvantageous that the layers which form from such pastes cannot ensure protection of the cathode from Cr evaporation from an interconnector through the air flowing through the cathode space.        